1. Field of the Invention
The present invention relates to an image formation process. More specifically, the invention relates to an image formation process in a copier or a printer based on an electrophotographic system. In particular, the invention relates to an image formation process by improving the step of development in the electrophotographing process. If described in further detail, the invention is concerned with a nonmagnetic one-component developing process, using an electric charger employing an electrically conducting brush, featuring excellent image properties such as resolution, gradation, image density and color reproduceability, as well as a developing agent used for the developing process. The invention further deals with a full-color nonmagnetic one-component developing method employing the above-mentioned system and a full-color developing agent used for the developing method.
2. Description of the Related Art
As is well known, an image-forming machine such as an electrophotographic copier, an electrophotographic printer or the like machine, is constituted by an electric charger for imparting a photosensitive property to an electrostatic recording medium, an exposure device for exposing an image carrier (electrostatic recording medium) to image-bearing light to form and record an electrostatic latent-image, a developing device which causes the electrostatic latent image recorded on the image carrier to electrically attract the developing agent to physically visualize the electrostatic latent image, an image transfer device for transferring the visualized image on the image carrier onto a recording medium such as a paper to record the image, and an image-fixing device for heating the image transferred onto the recording medium, to fix the image.
Further, a corona charger utilizing a corona discharge has been extensively used as an electric charger for uniformly charging the image carrier prior to the step of forming an electrostatic latent image on the image carrier. However, the corona charger requires the application of a voltage of as high as several kilovolts for effecting the corona discharge, resulting in an increase in the cost of producing the machine. Besides, ozone generated by the electric discharge damages the parts constituting the device and, particularly, shortens the life of the electrostatic recording medium. Moreover, an offensive odor due to ozone generated by the electric discharge causes discomfort to a person who uses the machine. Furthermore, ozone itself is harmful to human body when its concentration is high and adversely affects the environment.
In order to solve the above-mentioned problem, there has been proposed a so-called electrically conducting brush charger which is a charger which uses an electrically conducting brush as charging means instead of the corona charger. Referring to FIG. 1 which will be referred hereinafter to illustrates the present invention, the electrically conducting brush charger is one in which the electrically conducting brush 2 to which a voltage is applied from a power source 3 is rotated in contact with the image carrier 1 such as the dielectric member to electrically charge it, and is capable of electrically charging the image carrier 1 up to a required potential by applying a voltage of 500 to 1500 volts to the electrically conducting brush 2. Besides, the electrically conducting brush charger is free from the problem of generating ozone. The electrically conducting brush 2 is obtained by implanting brush-like electrically conducting fibers (e.g., rayon fibers) 5 on the periphery of an electrically conducting core rod 6. Further, electrically conducting brush chargers include fixed brush chargers which effect the electric charging using fixed plate-like electrically conducting brushes in addition to those which effect the electric charging while rotating a brush in the shape of a roller, as shown. The fixed brush electric charger can be realized in a small size and at a decreased cost.
However, the charging system using the electrically conducting brush involves defects caused by contamination in the brush-like fiber. That is, the brush-like fiber is in a state of in contact with the image carrier (e.g., photosensitive material drum). When the electrically conducting brush charger is repetitively used for extended periods of time, therefore, fouling on the photosensitive material drum such as residual toner, part of the toner component or paper dust adheres onto, or is adsorbed by, the brush fiber and accumulates. When the fouling exceeds a permissible level, the normal electric charge is reduced. This is because the electrically conducting brush relies upon a local electrical charging mechanism such as aerial discharge in a very small gap relative to the photosensitive material drum. In a portion where the brush fiber is contaminated, fine electric discharge does not take place, and it becomes difficult to realize a uniform and stable electric discharge, causing the electric discharge to occur irregularly. This is further caused by a frictional charge on the brush-like fiber relative to the photosensitive material drum and the injection of an electric charge from the brush-like fiber. In a practical image, the brush fiber produces irregularly swept portions, and background fogging occurs on the irregularly swept portions.
In an image formation process using an ordinary analog system, relatively nonuniform charging (irregular charging) on the photosensitive material drum corresponds to a black portion that is saturation-developed and does not become much of a problem. In a digital system employing reversal development, however, this corresponds to the background portion and could become a cause of background fouling. Besides, an improvement in the particulate property (decrease in the image noise) in a high-light portion (low-density region) which is a viewing point in reproducing natural image, becomes important if high image quality is demanded.
In addition, in the contact charging system using an electrically conducting brush, components and foreign matter contained in the developing agent and in the transfer paper adhere onto the brush causing a change in the latent image potential and developing relatively conspicuous background fouling.
Further, when a nonmagnetic one-component developing system is employed, the irregular charging on the photosensitive material becomes further conspicuous after developing. The reasons will be described hereinbelow.
The nonmagnetic one-component developing method using a nonmagnetic one-component developing agent (hereinafter also referred to as xe2x80x9cnonmagnetic tonerxe2x80x9d) can be executed in a manner as described below by using the developing device shown in FIG. 2 which can be used even in the embodiment of the present invention after the constitution has been improved. In the developing device 10, a developing roller 14 is provided in contact, under pressure, with a photosensitive material drum 1 in a toner container 13 installed near the photosensitive material drum 1 and, besides, a toner-replenishing roll 15 is provided in contact with the developing roller 14. The developing roller 14 is supplied with a developing bias from a power source 17. A one-component developing agent 11, which is a nonmagnetic toner, is contained in the toner container 13. Due to the rotation of the toner-replenishing roll 15, the toner 11 supplied onto the surface of the toner-replenishing roll 15 is conveyed to the surface contacting the developing roller 14. The non-magnetic toner 11 is formed in a thin layer, due to a toner layer thickness-limiting blade 16 that is in contact with the developing roller 14, and is conveyed to the photosensitive material drum 1 through the rotation of the developing roller 14. As a result, the electrostatic latent image on the surface of the photosensitive material drum 1 is developed with the nonmagnetic toner conveyed to the photosensitive material drum 1.
According to this developing method, it is important to form a nonmagnetic toner layer having a uniform and small thickness on the developing roller. Therefore, the layer thickness-limiting blade is provided to make uniform the thickness of the nonmagnetic toner layer adhered to the surface of the developing roller.
In this case, the electric charge to the toner varies depending upon the compression and friction between the developing roller and the toner-replenishing roller and between the developing roller and the layer thickness-limiting blade. Therefore, the electric charge on the toner is greatly affected by the surface coarseness of the developing roller. In particular, a very fine image is not reproduced in developing the toner for the latent image that is affected by the irregular electric charge when the electrically conducting brush is used.
Further, when the conventional toner having a nearly amorphous particle shape and a wide particle size distribution is used, a uniform and thin toner layer is not formed between the developing roller and the toner-replenishing roller or between the developing roller and the layer thickness-limiting member. Therefore, a fine half-tone image is not reproduced when developing the toner for a latent image that is affected by irregular electric charge when the electrically conducting brush is used.
Furthermore, the nonmagnetic one-component full-color process which successively executes developing many times and transfers them onto the transfer medium in an overlapped manner, is more strongly affected, and is not capable of forming a very fine full-color image like a printed image.
When the nonmagnetic one-component developing system is employed, the fouling of a brush and the resulting irregular charging stem not only from the electrically conducting brush that is used. The toner used as a developing agent in the image formation process and, particularly, the toner employing the nonmagnetic one-component developing system, is blended with various externally added agents such as inorganic fine particles for controlling the fluidity and charging properties of the toner. Here, the present inventors have discovered that these externally added agents cause fouling of the brush and irregular charging.
The externally added agents having a relatively large particle diameter (usually having a particle diameter of not smaller than 0.1 xcexcm) used for the nonmagnetic one-component developing agent, work to maintain long-lasting and stable fluidity and electrical charging properties. The reasons will now be described.
As will be understood from the above description of the nonmagnetic one-component developing method with reference to FIG. 2, the toner that is used as the developing agent in many cases receives mechanical pressure during the step of developing. Therefore, the externally added agent adhered to the surfaces of the toner particles is buried in the master toner constituted by a binder resin as a chief component, and the effect of the externally added agent is not exhibited to a sufficient degree. The externally added agent having a relatively large particle diameter is less likely to be buried than the externally added agent having a small particle diameter, and it is expected that long-lasting and stable fluidity and electrical charging properties are obtained.
In the two-component developing method using the carrier and the toner in combination, too, the externally added agent having a large particle diameter works advantageously. This is because the externally added agent forms protrusions which grind a film of spent toner formed on the carrier. The same effect is obtained even for the film formed on the photosensitive material drum. Further, when the toner image is to be transferred from the photosensitive material drum onto a recording medium such as paper, the externally added agent having a large particle diameter works to lower the adhering force of the toner to the photosensitive material drum, from which an improved transfer efficiency can be expected. Thus, the externally added agent having a large particle size in many cases works advantageously in the image formation process.
When the master toner and the externally added agent are mixed together without applying a sufficient degree of physical force, however, the externally added agent having a large particle diameter is simply adhered electrostatically to the master toner particles and can be easily peeled off the toner particles by the application of a physical force or an electric attractive force. It is considered that in the externally added agent having a small particle diameter, the Van der Waals force works dominantly; i.e., the externally added agent that is once adsorbed is less likely to be peeled off. The externally added agent having a large particle diameter, on the other hand, does not exhibit the Van der Waals force and may be liberated. Therefore, the free externally added agent having a large particle diameter induces brush fouling and causes irregular electric charging.
It is an object of the present invention to provide an image formation process which suppresses irregular electric charging of the electrically conducting brush as much as possible in the nonmagnetic one-component developing system equipped with an electric charger that uses the electrically conducting brush.
Another object of the present invention is to provide a toner for electrostatic charge developing in the nonmagnetic one-component developing method which is less affected by the irregular electric-charge of the electrically conducting brush and an image formation process.
A further object of the present invention is to provide a toner for full-color electrostatic charge developing based on the nonmagnetic one-component developing method that is less affected by the irregular electric charge of the electrically conducting brush despite the development being conducted many times successively and the images are transferred onto the transfer medium in an overlapped manner, and a full-color image formation process.
A still further object of the present invention is to provide an image formation process which suppresses the irregular electric charge as much as possible as a result of suppressing the fouling of the electrically conducting brush yet maintain a long life and high transfer efficiency by using a toner to which is added an externally added agent (external additive) having a relatively large particle diameter at the time of forming the image by using an image formation apparatus equipped with an electric charger using the electrically conducting brush.
A yet further object of the present invention is to provide a developing agent for electrophotography which is effective in putting the above-mentioned image formation process into practice.
The other objects of the present invention will be appreciated from the description as set forth below with regard to the preferred embodiments thereof.
The present inventors have discovered that a toner having a particular circularity forms a uniform and thin layer on a developer roller in the nonmagnetic one-component developing system and permits the image quality to be less affected by the irregular electric charge caused by the electrically conducting brush, and have arrived at the present invention.
The inventors have further discovered that the latent image written onto the photosensitive material does not make an irregular electrostatic latent image become conspicuous when the dot diameter lies within a particular range and, particularly, a half-tone image can be uniformly reproduced. Usually, the exposure spot is a circular shape or an oval shape. Therefore, the writing dot diameter referred to here stands for a diameter on the short axis thereof. When the diameter of exposure is changed by power modulation, the writing dot diameter stands for a maximum diameter (full dot diameter).
In dealing with the nonmagnetic one-component developing system using the electrically conducting brush, further, the inventors have discovered that a uniform and thin layer of toner is formed as the surface of the developing roller becomes very smooth, and a uniform image with a high resolution is obtained as the toner with the diminished reverse charge is used in the developing.
In dealing with the nonmagnetic one-component developing system using the electrically conducting brush, further, the inventors have discovered that the electric charge is easily introduced from the brush fiber to the toner and the electric charge quickly rises upon the use of an electrically conducting brush of a particular material and a brush fiber having a high density, making it possible to effectively suppress irregular electric charges on the photosensitive material.
In dealing with the nonmagnetic one-component developing system using the electrically conducting brush, further, the inventors have discovered that a proper developing amount is obtained when the toner on the developing roller has a particular amount of electric charge, and there is obtained an image of a suitable density, with little fogging, unaffected by an irregular electric charge on the brush.
In dealing with the nonmagnetic one-component developing system using the electrically conducting brush, further, the inventors have also discovered that a proper developing amount is obtained since the toner on the developing roller possesses a relatively narrow particle size distribution containing fine powders and coarse powders in amounts within a particular range, and there is obtained an image, of a suitable density and of a high resolution, unaffected by irregular electric charge of the brush.
In dealing with the nonmagnetic one-component developing system using the electrically conducting brush, further, the inventors have also discovered that a half-tone image is uniformly reproduced when an image is formed by using a full-color toner having a particular circularity, effecting the developing many times, and successively transferring the image onto the transfer medium.
According to the present invention, therefore, there is provided an image formation process wherein, in forming an electrostatic latent image according to an electrophotographic method and in forming a toner image by visualizing the electrostatic latent image with a developing agent, an electrically conducting brush is used as electric charging means, the brush which is impressed with a voltage is brought into contact with an image carrier to execute uniform electric charging, the electrostatic latent image is formed on the surface of the image carrier by exposure to the image-bearing light, and the toner image is formed by a nonmagnetic one-component developing system using a developing agent of a toner having a circularity within a range of from 0.92 to 0.98.
The present invention further provides a developing agent for electrophotography comprising a toner having a circularity over a range of from 0.92 to 0.98, the developing agent for electrophotography being used in an image formation process which forms the toner image by the nonmagnetic one-component developing system by bringing an electrically conducting brush impressed with a voltage into contact with a photosensitive material to effect uniform charging, and forming an electrostatic latent image on the photosensitive material by exposure to the image-bearing light.
The inventors have further studied the above-mentioned problems, and have discovered that the picture quality is less affected by irregular electric charge caused by the fouled brush as a result of optimizing the condition of the externally added agent added to the toner, and that the picture quality is less affected by irregular electric charge caused by the fouled brush as a result of optimizing the condition of the electrically conducting brush used by the electric charger, and have thus accomplished the present invention.
According to the present invention, therefore, there is provided an image formation process, wherein, in forming an electrostatic latent image according to an electrophotographic method and in forming a toner image by visualizing the electrostatic latent image with a developing agent, an electrically conducting brush is used as electric charging means, the brush which is impressed with a voltage is brought into contact with an image carrier to execute uniform electric charging, the electrostatic latent image is formed on the surface of the image carrier by the exposure to the image-bearing light, and the toner image is formed by using a developing agent of a toner containing at least an externally added agent having an average particle diameter over a range of from 0.1 to 2.0 xcexcm and a freeing ratio (separation ratio) of not larger than 20%.
The invention further provides a developing agent for electrophotography comprising a toner and containing at least an externally added agent having an average particle diameter over a range of from 0.1 to 2.0 xcexcm and a freeing ratio of not larger than 20%, the developing agent for electrophotography being used in an image formation process which forms the toner image by a nonmagnetic one-component developing system by bringing an electrically conducting brush impressed with a voltage into contact with a photosensitive material to effect uniform charging, and forming an electrostatic latent image on the photosensitive material by the exposure to image-bearing light.
The invention provides an image formation process improving such defects as background fouling, lack of image density or insufficient reproduction of a very fine image, caused by an irregular electric charge, by the charger using an electrically conducting brush in the nonmagnetic one-component process, compared to the prior art. The invention further provides a toner for high picture quality and a toner for full color adapted to the system.
In forming the image by using the image formation apparatus equipped with the electric charger using the electrically conducting brush of the present invention, further, irregular electric charge is suppressed as much as possible by suppressing the fouling of the electrically conducting brush yet extending the life or maintaining a high transfer efficiency by using a toner to which is added an externally added agent having a relatively large particle diameter, and the background fouling caused by the fouled brush is prevented. The invention further provides a color image formation apparatus of high performance by utilizing such distinguished effects.